Submerged membranes are used to treat liquids containing solids to produce a filtered liquid lean in solids and an unfiltered retentate rich in solids. For example, submerged membranes are used to withdraw substantially clean water from wastewater and to withdraw potable water from water from a lake or reservoir.
The membranes are generally arranged in modules which comprise the membranes and one or more headers attached to the membranes. The modules are immersed in a tank of water containing solids. A transmembrane pressure is applied across the membrane walls which causes filtered water to permeate through the membrane walls. Solids are rejected by the membranes and remain in the tank water to be biologically or chemically treated or drained from the tank.
Air bubbles are introduced to the tank through aerators mounted below the membrane modules and connected by conduits to an air blower. The air bubbles rise to the surface of the tank water and create an air lift which recirculates tank water around the membrane module. When the rate of air flow is within an effective range, the rising bubbles and tank water scour and agitate the membranes to inhibit solids in the tank water from fouling the pores of the membranes. Further, there is also an oxygen transfer from the bubbles to the tank water which, in wastewater applications, provides oxygen for microorganism growth. The air blower generally runs continuously to minimize stress on the air blower motors and to provide a constant supply of air for microorganism growth if desired.
With typical aeration systems, an operator increases the rate of air flow to the aerators if more cleaning is desired. This technique, however, stresses the membranes and air blower motors and increases the amount of energy used which significantly increases the operating costs of the process. Conversely, an operator typically decreases the rate of air flow to the aerators if less cleaning is desired. With this technique, however, the rate of air flow is often below the effective range, which does not provide efficient cleaning. Alternately, some operators reduce the average rate of air flow by providing air intermittently. This method allows for an air flow rate in the effective range but at the expense of the air blowers which wear rapidly when turned off and on frequently. In many cases, the warranty on the air blower is voided by such intermittent operation.
Another concern with typical aeration systems is that they cause the tank water to move in a generally steady state recirculation pattern in the tank. The recirculation pattern typically includes “dead zones” where tank water is not reached by the recirculating tank water and bubbles. The membranes in these dead zones, or the parts of the membranes in these dead zones, are not effectively cleaned and may be operating in water having a higher concentration of solids than in the tank water generally. Accordingly, these membranes, or the affected parts of these membranes, quickly foul with solids.
A related problem occurs in modules where hollow fibre membranes are installed with a small degree of slack to allow the membranes to move and shake off or avoid trapping solids. The movement of tank water in the tank encourages slackened membranes to assume a near steady state position, particularly near the ends of the membranes, which interferes with the useful movement of the fibres.
Yet another concern with current aeration systems is that the aerators themselves often foul over time. Even while the air supply is on, the local air pressure near the perimeter of the aerator holes is low and often allows tank water to seep into the aerator. When aeration is stopped from time to time, for example for backwashing, cleaning or other maintenance procedures, more tank water may enter the aeration system. A portion of the tank water entering the aeration system evaporates there, leaving deposits of solids in the aeration system. In wastewater applications in particular, the deposited solids can significantly reduce the efficiency of the aeration system or cause an operator to periodically shut down filtration to clean or replace the aerators.